TRANSFORMERLESS HIGH STEP-UP DC-DC COCKCROFT- WALTON VOLTAGE MULTIPLIER FOR A HYBRID SYSTEM APPLICATION 1 CHEERU G. SURESH, 2 ELIZABETH RAJAN, 3 CHITTESH V.C., 4 CHINNU G. SURESH 1,3 PG Student, Saintgits College of Engineering 2 Assistant Professor, Saintgits College of Engineering 4 PG student, Sree Narayana Gurukulam college of Engineering Abstract- In a hybrid system of photo voltaic and fuel cell, the fuel cell can be kept as a back-up source replacing a battery. The hydrogen for the fuel cell can be produced from an electrolyser which runs by the excess solar power during the off peak hours. The hydrogen fuel can be stored in a storage tank and can be used during the absence of solar energy. The low output voltages of solar and fuel cell can be boosted by a high step up dc-dc converter based on a Cockcroft-Walton(CW) voltage multiplier without a step-up transformer. A CW converter has a very high voltage gain. This converter operates in continuous conduction mode (CCM), so that the switch stresses can be reduced. By increasing the number of stages, higher values for output voltage can be obtained. A 5-stage simulation of a Cockcroft-Walton voltage multiplier is conducted and the result shows a high stepped up value of output voltage. Keywords- Hybrid system, Fuel cell, Electrolyser, Cockcroft-Walton multiplier, Continous conduction mode. I. INTRODUCTION Wide use of electrical equipment leads to the increased use of electrical energy. This leads to the increased demand of renewable sources. Photovoltaic (PV) cells and Fuel cells are attractive choices. But the outputs obtained by these are at low level. Thus a high step-up of the output is required. Among all renewable sources solar PV is an attractive choice since the greenhouse gas emission is less. The module efficiency is dependent on the insolation, shadow, temperature, characteristics of the spectrum and many other factors. Also due to the lack of its availability during the night time we need to store solar energy somehow. Battery storage system can be preferred. But considering the life and gas emission it is less preferred. So a fuel cell is a better choice. The main fuels for fuel cells are hydrogen (or hydrogen rich fuels) and oxygen. The hydrogen fuel can be produced by using an electrolyser and can be stored in a tank. After all the output of these cells, both solar and fuel are much less. elements depends on the number of stages. Researches for a converter which provides high voltage ratio, low voltage stress on diodes and capacitors, compactness and cost efficiency leads to the wide use of CW voltage multiplier. In section II the modelling of components in the system is explained. An n-stage CW converter, its design consideration, operating principle and control strategy of the proposed converter is explained in section III. In section IV the simulation and experimental results were displayed for verification. And finally some conclusions are given in section V. II. HYBRID SYSTEM MODELLING The system components consists of a solar PV generator, a Fuel Cell, an electrolyser, a hydrogen storage tank, power conditioning unit and load. For stepping up of these output voltages we need to boost its voltages for practical purposes. Theoretically a very high voltage gain can be produced by a conventional boost converter by an extremely high duty cycle. By using an isolated transformer or coupled inductors we can obtain a high step up value without high duty cycle. Since these structures are complex some alternatives without transformers and coupled inductors were presented by cascading diodes and capacitors. These structures are simple and robust and provides high voltage gain. But the voltage stress on individual switch and passive Fig. 1. Photo Voltaic- Fuel Cell Hybrid energy system A. Solar Photo Voltaic Generator A combination of PV cells arranged in series or parallel or both to form a PV array. Basically a solar cell is a p-n junction fabricated in a thin wafer of semiconductors. The output voltage of a PV cell is a function of the photocurrent, which is determined by the load current depending on the solar irradiance during the operation. 84
The equivalent circuit of a PV cell is shown in Figure 2. The output power of a PV generator PPV is Fig. 2. Simulation Results B. Fuel Cell The fundamental structure of a PEM fuel cell consist of two electrodes(anode and cathode) separated by a solid membrane which acts as an electrolyte. Fig. 3. Cross sectional view of a PEMFC A PEMFC will convert the chemical energy of a fuel like hydrogen into a clean and efficient electrical energy with only water and heat as its bye-products. The electrochemical model of fuel cell is based the equation, E. Power Conditioning Unit The power conditioning unit consists of a power controller and DC-DC Cockcroft-Walton voltage multiplier. The power controller compares the load requirement and the power generated by the PV module. If the load requirement is more it has to control the flow rate of hydrogen and oxygen to the fuel cell to generate the excess power so as to meet the load. The Cockcroft-Walton multipier circuits consists of a volt-age multiplier ladder network of capacitors and diodes to generate high voltages. Using only capacitors and diodes, these voltage multipliers can step up relatively low voltages to extremely high values, while at the same time being far lighter and cheaper than transformers. It has the advantage of requiring relatively low cost components and being easy to insulate. Each stage consists of two diodes and two capacitors. One can also tap the output from any stage, like a multi-tapped transformer. A CW multiplier normally works for an alternating current. In order to obtain an alternating current, the input dc will be controlled by four switches. As the number of stages are increasing higher values for output voltage will be obtained. III. N STAGE CW VOLTAGE MULTIPLIER In this paper the boost type structure helps to provide higher voltage ratio than the conventional Cockcroft- 85
Walton voltage multiplier. Thus this converter can be used for power conversion application where high voltage gains are desired. Moreover, this converter is operated in continuous conduction mode (CCM) and hence the switch stress and switching losses can be well reduced. The diagram of a three stage converter is shown in Figure 4. Fig. 4. Converter with n-stage CW voltage multiplier. A. Design considerations of cockcroft-walton converter 1) voltage stress on capacitors: Theoretically in an n stage Cockcroft-Walton voltage multiplier, all the capacitors except the first one has same voltage. The first capacitor will have only the half the voltage of the other. The capacitor voltage depends only on the input voltage and its duty cycle, not the number of stages. Thus determination of capacitor rating is easier of this converter. 86
Fig. 5. Conducting paths of proposed converter. (a) State I. (b) State II-A. (c) State II-B. (d) State II-C. (e) State III. (f) State IV-A. (g) State IV-B. (h) State IV-C. C. Control strategy of n stge Cockcroft-Walton Voltage Multiplier In this paper the boost type structure helps to provide higher voltage ratio than the conventional CW voltage multiplier. Thus this converter can be used for power conversion application where high voltage gains are required. Moreover this converter is operated in continuous conduction mode(ccm) and hence the switch stress and switching losses can be well reduced. Since the input source is a dc voltage source, a pulsating current is obtained at the CW voltage multiplier by using four switches Sm1,Sm2,Sc1,Sc2. Sm1 and Sm2 works at modulating frequency (fsm) and Sc1 and 87
Sc2 works at alternating frequency (fsc). Sm1 (Sc1) and Sm2 (Sc2) operates in a complementary mode. IV. SIMULATION AND RESULTS The mathematical model and control strategy of the pro-posed converter is simulated by using MATLAB/Simulink model. The simulaton is carried out for a 5-stage CWconverter. An output DC voltage of 250V is expected for an input voltage of 21V DC. Fig. 6. Converter with n-stage CW voltage multiplier. CONCLUSION Since the hybrid system consist of mainly solar PV generator and fuel cell technology the output will be only electrical energy, water and heat. So the system is ecofriendly. The presence of fuel cells replacing batteries helps to improve the life of the system. The use of electrolyser cells helps to collect the solar energy and to store it for the production of electrical energy in the absence of solar power. The cockcroftwalton voltage multiplier proposed in this paper avoids the use of step up transformer and thus reduces the cost and bulkiness. The two independent frequencies, high frequency helps to reduce the inductor size and low frequency is determined by the desired output voltage ripple. The future work includes a power controller to control the generated power by considering the load requirement. 88 REFERENCES [1] G. R. Walker and P. C. Sernia, Cascaded DCDC Converter Connection of Photovoltaic Modules, IEEE Trans. Power Electron., vol. 19, no. 4, pp. 11301139, Jul. 2004. [2] J. Wang, S. Member, F. Z. Peng, S. Member, J. Anderson, and A. Joseph, Low Cost Fuel Cell Converter System for Residential Power Generation, vol. 19, no. 5, pp. 13151322, 2004. [3] W. Xiao, S. Member, W. G. Dunford, S. Member, P. R. Palmer, and A. Capel, Regulation of Photovoltaic Voltage, vol. 54, no. 3, pp. 13651374, 2007. [4] F. Liu, S. Duan, F. Liu, B. Liu, and Y. Kang, A Variable Step Size INC MPPT Method for PV Systems, IEEE Trans. Ind. Electron., vol. 55, no. 7, pp. 26222628, Jul. 2008. [5] M. A. Al-refai, Matlab / Simulink Simulation of Solar Energy Storage System, no. 2, pp. 304309, 2014. [6] R. Wai, S. Member, C. Lin, and C. Lin, High-Efficiency Power Con-version System for Unit With Low Input Voltage, vol. 55, no. 10, pp. 37023714, 2008. [7] J. Kwon, E. Kim, B. Kwon, and K. Nam, High-Efficiency Fuel Cell Power Conditioning System With Input Current Ripple Reduction, IEEE Trans. Ind. Electron., vol. 56, no. 3, pp. 826834, Mar. 2009. [8] A. K. Rathore, A. K. S. Bhat, R. Oruganti, S. Member, and A. Abstract, Analysis, Design and Experimental Results of Wide Range ZVS Active- Clamped L-L Type Current-Fed DC / DC Converter for Fuel Cells to Utility Interface, vol. 59, no. 1, pp. 473485, 2012. [9] B. Yuan, S. Member, X. Yang, X. Zeng, J. Duan, J. Zhai, and D. Li, Analysis and Design of a High Step-up Current- Fed Multiresonant DC DC Converter With Low Circulating Energy and Zero-Current Switching for All Active Switches, vol. 59, no. 2, pp. 964978, 2012. [10] R. Wai, S. Member, C. Lin, R. Duan, and Y. Chang, High- Efficiency DC-DC Converter With High Voltage Gain and Reduced Switch Stress, vol. 54, no. 1, pp. 354364, 2007. [11] T. Wu, S. Member, Y. Lai, J. Hung, and Y. Chen, Boost Converter With Coupled Inductors and Buck Boost Type of Active Clamp, vol. 55, no. 1, pp. 154162, 2008. [12] S. Lee, S. Member, S. Rhee, and G. Moon, Coupled Inductor Incorporated Boost Half-Bridge Converter With Wide ZVS Operation Range, vol. 56, no. 7, pp. 25052512, 2009. [13] C. Leu, P. Huang, S. Member, and M. Li, A Novel Dual- Inductor Boost Converter With Ripple Cancellation for High-Voltage-Gain Applications, vol. 58, no. 4, pp. 12681273, 2011. [14] O. Abutbul, A. Gherlitz, Y. Berkovich, A. Ioinovici, and S. Member, Step-Up Switching-Mode Converter With High Voltage Gain Using a Switched-Capacitor Circuit, vol. 50, no. 8, pp. 10981102, 2003. [15] F. L. Luo and H. Ye, Positive Output Multiple-Lift Push Pull, vol. 51, no. 3, pp. 594602, 2004. [16] F. L. Luo and H. Ye, Positive output cascade boost converters, vol. 151, no. 5, 2004. [17] B. Axelrod, Y. Berkovich, and A. Ioinovici, Structures for Getting Transformerless Hybrid DC DC PWM Converters, vol. 55, no. 2, pp. 687696, 2008. [18] Y. Berkovich, B. Axelrod, and a. Shenkman, A novel diodecapacitor voltage multiplier for increasing the voltage of photovoltaic cells, 2008 11th Work. Control Model. Power Electron., pp. 15, Aug. 2008.
[19] N. D. C. D. C. Converters, M. Prudente, L. L. Pfitscher, G. Emmen-doerfer, E. F. Romaneli, and R. Gules, Voltage Multiplier Cells Applied to, vol. 23, no. 2, pp. 871887, 2008. [20] I. C. Kobougias and E. C. Tatakis, Optimal Design of a Half-Wave CockcroftWalton Voltage Multiplier With Minimum Total Capacitance, IEEE Trans. Power Electron., vol. 25, no. 9, pp. 24602468, Sep. 2010. [21] M. D. Bellar, E. H. Watanabe, and a. C. Mesquita, Analysis of the dynamic and steady-state performance of Cockcroft- Walton cascade rectifiers, IEEE Trans. Power Electron., vol. 7, no. 3, pp. 526534, Jul. 1992. [22] F. Hwang, Y. Shen, S. H. Jayaram, and S. Member, Low- Ripple Compact High-Voltage DC Power Supply, vol. 42, no. 5, pp. 11391145, 2006. [23] N. Pandiarajan and R. Muthu, Mathematical Modeling of Photovoltaic Module with Simulink, no. Icees, pp. 35, 2011. [24] P. E. M. F. Cell, Modeling and Design of Fuel Cell based Two Phase Interleaved Boost Converter, pp. 7277, 2011. [25] Z. Ural, M. T. Genolu, and B. Gm, Dynamic Simulation of a Pem Fuel Cell System, no. July, pp. 1315, 2007. [26] C. Rani, S. Sriganesh, and B. Muralikrishnan, Dynamic modeling and control of a wind turbine generator with fuel cell, Ultra capacitor stack as an auxiliary storage, no. 1, 2011. [27] S. D. C. D. C. Converter, C. Young, M. Chen, and S. Member, Cascade Cockcroft Walton Voltage Multiplier Applied to Transformerless High, vol. 60, no. 2, pp. 523537, 2013. 89